The new default editor of WordPress currently available in a form of plugin, as it is still in development — not yet 100% ready at all. However its major user interface/experience and native blocks are nearly complete.
Instead of installing directly in your site to test it out, testgutenberg.com will bring you to an instant playground. Because it almost be able to do the same as it’s integrated in your own WordPress site. The only limitation is media upload due to disk space consideration. It also doesn’t allow to save the post in order to make database cleaner. To preview final editing, what you need to do is toggling the hamburger menu at the top left near the logo.
This tool was made by Tom J Nowell. He wrote a post about how it was developed in his blog. I’d say thanks for the brilliant work. I am sure it will extremely help better understand the mechanism of composing a post on new editor. I also feel that it’s signaling us we’re getting closer to WordPress 5.0, where Gutenberg will be merged into core.